Apparatus for forming images, method of controlling the apparatus, and storage medium

ABSTRACT

The invention intends to relatively increase resolution when the size of an image to be output for recording is small, enabling even a gradation image to be visually perceived with satisfactory quality. To this end, in a disclosed image forming apparatus for copying an image of an original document set on a reading apparatus, when the size of an image to be output is not larger than a predetermined size, a CPU controls a resolution switching section to increase resolution, thereby forming the image with the increased resolution.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an apparatus for forming images,a method of controlling the apparatus and a storage medium. Moreparticularly, the present invention relates to an image formingapparatus for forming a visible image on a predetermined recordingmedium, a method of controlling the apparatus, and a storage mediumstoring a program used for forming such a visible image.

[0003] 2. Description of the Related Art

[0004] An electrophotographic printer will be described as one exampleof conventional image forming apparatus.

[0005] First, the electrophotographic printer employs, as an imagesignal, a signal with resolution of 400 dpi. Then, a gradation imagereproducing section of the printer sets resolution of an output imageunder conditions favorable for reproducing intermediate tones, i.e., 200dpi in the direction of main scan (direction in which an image isscanned by a laser) and 400 dpi in the direction of sub-scan (directionperpendicular to the direction of main scan). On the other hand, acharacter/line image reproducing section of the printer sets resolutionof an output image under conditions favorable for reproducing characteredges, i.e., 400 dpi in the direction of main scan and 400 dpi in thedirection of sub-scan.

[0006] Further, in certain copying machines, an image is reproduced withpixel resolution of 200 dpi in the direction of main scan and 400 dpi inthe direction of sub-scan in a mode where an emphasis is placed ongradation, such as a printed photo mode or a photographic paper photomode, and with pixel resolution of 400 dpi in the direction of main scanand 400 dpi in the direction of sub-scan in a mode where an emphasis isplaced on edge reproduction of character/line images, such as acharacter mode or a map mode.

[0007] Additionally, in a character/photo mode, an edge portion ofcharacters and thin lines is determined as to whether it represents anarea of a gradation image or a character/line image. Then, the areadetermined as being a gradation image is reproduced with pixelresolution of 200 dpi in the direction of main scan and 400 dpi in thedirection of sub-scan, while the area determined as being acharacter/line image is reproduced with pixel resolution of 400 dpi inthe direction of main scan and 400 dpi in the direction of sub-scan.

[0008] Even for a gradation image, however, persons tend to very oftenlook at the image from a close distance when the image size is small.

[0009] This raises a problem that when the resolution is lowered byreason of a gradation image as mentioned above, the contours of dots nowbecomes conspicuous and image quality is deteriorated contrary to theintention. In particular, when the image is observed at the distance ofabout 15 cm from the image to the viewer's eye, the presence of each dotis visually recognized and such a problem is more remarkable.

SUMMARY OF THE INVENTION

[0010] In view of the above-stated problem in the art, an object of thepresent invention is to provide an apparatus for forming images, amethod of controlling the apparatus, and a storage medium, with whichpixel resolution is relatively raised when the size of an image to beoutput for recording is small, enabling even a gradation image to bevisually perceived with satisfactory quality when observed from a closedistance.

[0011] To achieve the above object, an apparatus for forming imagesaccording to the present invention is constructed as follows.Specifically, in an image forming apparatus for forming a visible imageon a predetermined recording medium based on input digital image data,the apparatus comprises image forming means capable of forming visibleimages with multiple degrees of resolution, and control means forcontrolling the image forming means to select one of the multipledegrees of resolution depending on the size of an image area to beformed by the image forming means.

[0012] In the above image forming apparatus, preferably, the controlmeans controls the image forming means to select one of the multipledegrees of resolution depending on the size of a recording medium. Withthis feature, optimum resolution can be simply set when an originalimage is recorded on a smaller scale, for example.

[0013] In the above case, preferably, the control means controls theimage forming means to select higher one of the multiple degrees ofresolution as the size of a recording medium is reduced. With thisfeature, even when a printed image is observed from a close distance,the contours of dots become less conspicuous and the image can bevisually recognized with good quality.

[0014] The image forming apparatus may further comprise document readingmeans, and designating means for designating an effective area in adocument read by the document reading means, the control meanscontrolling the image forming means to select one of the multipledegrees of resolution depending on the size of the effective areadesignated by the designating means. In this case, preferably, thecontrol means controls the image forming means to select higher one ofthe multiple degrees of resolution as the effective area designated bythe designating means is reduced.

[0015] Alternatively, the image forming apparatus may further comprisedocument reading means, and preparing means for preparing a histogram ofdensity distribution in a document read by the document reading means,the control means controlling the image forming means to select one ofthe multiple degrees of resolution depending on the density distributiongiven by the prepared histogram.

[0016] Other objects and forms of the present invention will be apparentfrom the following description made with reference to the drawings andthe attached claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a sectional view showing the construction of an imageforming apparatus according to a first embodiment.

[0018]FIG. 2 is a chart for explaining the principle of forming pixelsof 200 dpi in the direction of main scan in the first embodiment.

[0019]FIG. 3 is a chart for explaining the principle of forming pixelsof 400 dpi in the direction of main scan in the first embodiment.

[0020]FIG. 4 is a graph showing an MTF characteristic of the human senseof vision.

[0021]FIG. 5 is a perspective view showing an appearance of a copyingmachine according to a second embodiment.

[0022]FIG. 6 is a flowchart of processing steps according to a thirdembodiment.

[0023]FIG. 7 is a graph showing one example a histogram prepared in thethird embodiment.

[0024]FIG. 8 is a block diagram of a resolution switching section andthereabout in the first embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Preferred embodiments of the present invention will be describedin detail hereunder with reference to the accompanying drawings.

[0026] (First Embodiment)

[0027]FIG. 1 shows the sectional construction of an image formingapparatus, practiced as a copying machine, according to a firstembodiment. As shown, the copying machine comprises a reading apparatus23 and an image forming apparatus 24.

[0028] Various components of the reading apparatus 23 will be describedbelow in conjunction with operation.

[0029] An original document 1 put on a document glass plate 2 isilluminated by light delivered by a combination of a light source 3 anda reflecting mirror 4. The light reflected by the document 1 is bent inits optical path by a turning-back mirror 5 and then focused by anoptical lens 6 onto a CCD sensor (hereinafter abbreviated to CCD) 7 in aone-dimensional array. The light source 3, the reflecting mirror 4 andthe turning-back mirror 5 are scanned together as one unit in thedirection of arrow V to thereby take in a two-dimensional documentimage. Note that the reading apparatus 23 in this embodiment is capableof reading an original document up to the A3 (11 inch×17 inch) size atmaximum.

[0030] Users of the copying machine can set a desired paper size (sizeof an output medium), on which an original image is to be copied, from acontrol panel (not shown). In some cases, users can also automaticallyor manually set a zooming ratio so that an original image may be copiedin a desired paper size different from the original size. The zoomingratio is changed by controlling the speed V in the direction of mainscan of the document and by performing an interpolating or thinning-outprocess in the direction of sub-scan of the document.

[0031] Alternatively, the copying machine may be designed to select apaper size (size of an output medium) by using a well-known means forautomatically sensing the document size or in response to an instructionfrom the control panel (not shown).

[0032] An image signal produced by the CCD 7 is sent to an imageprocessing section 8 of the image forming apparatus 24.

[0033] The image processing section 8 converts an analog signal sentfrom the CCD 7 into a digital signal, carries out a predetermined imageprocess including y-correction, etc., and outputs image data to aresolution switching section 26. Based on the image data from the imageprocessing section 8, the resolution switching section 26 produces avideo signal (i.e., a signal for driving a semiconductor laser unit 9).Resolution of the image data produced at this time is controlled by aCPU 25 (as described later in detail).

[0034] In accordance with the applied video signal, the semiconductorlaser unit 9 emits a laser beam during a period in which the videosignal has a high level. The emitted laser beam is irradiated to andreflected by one side face of a rotating polygonal mirror 27 so that itis oscillated to make one-dimensional scan. The oscillated laser beam isreflected by a reflecting mirror 10 to scan the surface of aphotosensitive drum 11 for exposure.

[0035] The photosensitive drum 11 is rotating at a constant speed in thedirection of arrow R1 as shown. Around the photosensitive drum 11, thereare disposed a charger 12 for uniformly charging the surface of thephotosensitive drum 11, a developer 13 for depositing toner to anelectrostatic latent image formed by the exposure of the laser beam,thereby developing the latent image, a transfer device 14 fortransferring a toner image onto a sheet of recording paper, and acleaner 15 scraping off the toner remaining on the surface of thephotosensitive drum 11.

[0036] Sheets of recording paper (having different sizes from eachother) are stored in paper feed cassettes 18, 20. The CPU 25 drives apaper feed roller 17 or 19 to start transport of top one of the papersheets in the corresponding cassette, and also drives feed rollers 16 atthe predetermined timing, causing the sheet of recording paper to beinserted between the photosensitive drum 11 and the transfer device 14.As an alternative, a sheet of recording paper having a desired size maybe inserted through a manual feed inlet (not shown) so that an image isrecorded on the inserted sheet of recording paper.

[0037] As a result, the toner image is transferred onto the sheet ofrecording paper. The sheet of recording paper having the toner imagetransferred thereon is moved over a transport belt 21 to a fuser 22where the toner image is finally fused. After that, the sheet ofrecording paper is ejected out of the apparatus.

[0038] In the above-described embodiment of the image forming apparatuswhich is constructed to be capable of switching resolution, when thesize of a document to be output is not larger than a predetermined size(e.g., B5 size), an image is recorded with resolution (e.g., 400 dpi inboth the directions of main scan and sub-scan) higher than resolution(e.g., 200 dpi in the direction of main scan and 400 dpi in thedirection of sub-scan) that is usually employed when the document is anintermediate-tone image (photographic image). Of course, this switchingoperation is also performed even for an original document to be readhaving the A4 size, for example, if the original document is reduced toa size not larger than B5 when recorded.

[0039]FIG. 8 shows one example of the configuration of part of theresolution selecting section 26 and the CPU 25.

[0040] As shown in FIG. 8, the resolution switching section 26 receivesthe digital image data (e.g., 8 bits=256 gradations per pixel) processedby the image processing section 8 in synch with a pixel clock CLK. Thereceived data is converted into an analog signal by a D/A converter 80.The resulting analog signal is supplied to one input terminal of acomparator 81.

[0041] A pattern signal (triangular wave in this embodiment) generatedby one of triangular wave generators 82, 83 is supplied to the otherterminal of the comparator 81. A switch 84 is provided to select onetriangular wave and changed over under control of the CPU 25.

[0042] As a result, the comparator 81 outputs a signal (pulse widthmodulation signal) with a width corresponding to the value of thedigital pixel data. While the output signal of the comparator 81 istaking a high level, a semiconductor laser device is driven to emit alaser beam for a time corresponding to the value of the digital pixeldata. Because the laser beam is one-dimensionally oscillated by therotating polygonal mirror 27, the laser beam scans the surface of thephotosensitive drum 11 for exposure in a length corresponding to thevalue of the digital pixel data. Eventually, toner is deposited over anarea corresponding to the value of the digital pixel data, and personsvisually perceive the toner area as density of a reproduced image.

[0043] The triangular wave generators 82 and 83 differ from each otherin that the triangular wave generator 82 generates a triangular wave(for 200 dpi) at frequency equal to ½ of the carrier clock CLK, whereasthe triangular wave generator 83 generates a triangular wave (for 400dpi) at frequency equal to the carrier clock CLK.

[0044] This results in laser emission patterns as shown in FIGS. 2 and3, by way of example, when the triangular wave from the triangular wavegenerator 82 and the triangular wave from the triangular wave generator83 are selected, respectively.

[0045] Specifically, in the case of FIG. 2, formed dots are relativelylarge, which is favorable to reproduce a gradation image with a goodgradient. On the other hand, relatively small dots (with higherresolution) as shown in FIG. 3 are favorable to reproduce characters andline drawings. As explained before, however, persons tend to usuallylook at prints, etc. from a close distance when the print size is small.This means that, at resolution of 200 dpi, roughness of relatively largedots becomes conspicuous to such an extent as deteriorating imagequality even with a good gradient. For this reason, this embodiment isdesigned to record an image with resolution of 400 dpi in both thedirections of main scan and sub-scan when the size of a document to beoutput is not larger than the predetermined size.

[0046] In other words, when an image is formed on a small sheet of paperof not larger the predetermined size (B5 in this embodiment), theapparatus of this embodiment forms the image in a high-resolution mode,particularly in a mode where pixels are produced with a period not lessthan 111 lines/mm, i.e., 250 dpi, as shown in FIG. 4, so that a visuallypleasing image can be obtained.

[0047] More specifically, let now suppose, for example, that afull-length image of one person is formed on a sheet of paper of thepost card size. If such an image is reproduced with resolution of 200dpi in the direction of main scan and 400 dpi in the direction ofsub-scan as conventional, the pixel configuration of a reproduced imagewould be conspicuous when looking at the face of the person, and thereproduced image would have unsatisfactory reproducibility in itsdetails when viewed from a close distance. By contrast, in thisembodiment, the resolution in the direction of sub-scan is increased tosuch an extent that viewers cannot visually recognize the pixelconfiguration. As a result, a better-looking image can be obtained whichis reproduced finely and clearly up to its details.

[0048] In this embodiment, sheets of paper of not larger than the B5size are assumed to be small in size. But it is needless to say that thesize requiring the resolution switching operation is not generallyrestricted because the criteria used for judging whether a sheet ofpaper is small or not in size from the resolution point of view maychange depending on characteristics of individual image formingapparatus.

[0049] Incidentally, gradation reproducing characteristics in thisembodiment are adjustable in accordance with gradation convertingcharacteristics (i.e., a γ-lookup table) for each of modes independentlyso that the density of the reproduced image becomes equal to the densityof the original document in any of the high-resolution mode and theconventional gradation image reproducing mode.

[0050] It is also needless to say that this embodiment is applicable todigital full-color copying machines as well rather than restricted tomonochrome (black-and-white) copying machines.

[0051] Further, the above embodiment has been described in connectionwith the case of switching the resolution between two values (i.e., 400dpi and 200 dpi in the direction of main scan). However, when a printerengine has high resolution of 1200 dpi, the resolution may be changed inmultiple steps depending on the size of sheets of recording paper to beoutput. Specifically, in such a case, three or more triangulargenerators are prepared and switched over from one to anotherappropriately. It is of course desired for the CCD 7 to intrinsicallyhave high reading resolution. But, depending on cases, the readingresolution may be changed through an interpolating or thinning-outprocess.

[0052] While the resolution switching is realized in the aboveembodiment by selecting one of the two triangular wave generators,essentially the same operating advantage as described above can also beobtained by making variable the rotating speed of the rotating polygonalmirror and the pixel clock CLK. In the case of high-resolutionrecording, the number of pixels may be increased for the resolutionswitching by raising the reading resolution or performing aninterpolating process.

[0053] (Second Embodiment)

[0054] Generally, some models of digital full-color copying machinesinclude an editor having a coordinate input device, such as a digitizer,mounted on a presser plate for retaining a document in place, enablingusers to readily carry out various editing functions such as trimming,masking and color conversion.

[0055] A copying machine according to a second embodiment of the presentinvention includes, as will be seen from an appearance shown in FIG. 5,an editor comprised of a position coordinate designating pen 501 and abase 502 which is sensitive to pressure applied by pushing down the pen501 and able to determine position coordinate values of the pen. Userscan perform trimming of an original document by designating a certainarea on the base 502 as desired. In such a copying machine, an image isformed by automatically selecting a high-resolution mode when thetrimming size (area) is not larger than 200 cm², for example, and alow-resolution mode when the trimming size (area) is larger than 200cm². By so switching the resolution depending on the trimming size,quality of an image reproduced in small size format can be improved.

[0056] In this second embodiment, whether to effect the resolutionswitching or not is judged based on the trimming size, i.e., the size ofa document portion to be trimmed, and does not depend on the size of asheet of recording paper even when a trimmed image is to be output on asheet of recording paper of the A4 or larger size. The reason is that ifthe trimmed image is reproduced by full-size recording, the image sizeactually recorded is a portion of the total sheet size of recordingpaper and persons tend to eventually look at the recorded image from aclose distance.

[0057] (Third Embodiment)

[0058] In an image forming apparatus of a third embodiment, theapparatus automatically determines the size of a gradation image andselects one of a high-resolution mode and a low-resolution modedepending on the determined result.

[0059] The image forming apparatus is assumed here to have the sameconstruction as in the above first embodiment, and an operationprocessing procedure executed by the CPU 25 (i.e., a program stored inROM of the CPU 25) will be described below with reference to a flowchartof FIG. 6.

[0060] First, in step S61, a document image is read by prescan and ahistogram of pixel density is prepared.

[0061] The histogram is prepared by plotting the number of pixels basedon the pixel density ranging from 0 to 255 and dividing the entire rangeinto four regions; i.e., I region from 0 to 64, II region from 65 to128, III region from 129 to 192, and IV region from 193 to 255.

[0062] The histogram of FIG. 7 represents the case where an originaldocument includes a gradation image of small size.

[0063] In consideration of memory capacity, the histogram is prepared inthis embodiment based on the pixels thinned out to ¼ of the totalnumber. But the histogram may be of course prepared based on the totalnumber of pixels. It is to be here noted that a distribution of pixelsrepresenting the general feature of an image will not be entirelyeliminated by thinning out the pixels to ¼ of the total number. From thestandpoint of effectively utilizing a memory, therefore, it is desiredto execute a thinning-out process (i.e., to increase intervals at whichpixels are read).

[0064] Then, pixel percentages in the respective areas from I to IV(i.e., percentages of the numbers of pixels in the respective areas withrespect to the total pixel number) are determined and registered in amemory.

[0065] The process flow goes to step S62 to determine whether therelation of I Region+IV Region>80% is satisfied or not. If satisfied,then the document image is determined as containing characters at a veryhigh rate and a character mode (i.e., a mode with 400 dpi in thedirection of main scan, 400 dpi in the direction of sub-scan and anincreased sharpness factor (edge emphasis)) is set (step S63).

[0066] The meaning of increasing the sharpness factor will now bedescribed in brief.

[0067] Generally, for a binary image comprising characters/linedrawings, etc., edges of the image tend to blur if read data of theimage is utilized directly. Because it is essentially important thatedges of characters/line drawings be clearly reproduced at theirboundaries, that tendency produces a contrary result. This requires adensity value of the target pixel to be modified in such a manner thatthe target pixel is emphasized in relation to the surrounding pixels.Such a process of emphasizing the target pixel is usually executed byusing a matrix which has coefficients corresponding to pixels in anappropriate area, and modifying the coefficients of the matrix in asuitable manner for the emphasis.

[0068] Thus, increasing the sharpness factor in step S63 means that theCPU 25 makes setting adapted to perform a higher degree of edgeemphasis.

[0069] If the determined result in step S62 is NO, then the processflows goes to step S64 to determine whether the relations of I Region<IIRegion+III Region and I Region+II Region+III Region<15% are satisfied ornot. If satisfied, then it is judged that a gradation image is presentin the original document, but its size is small. Therefore, ahigh-precision mode is set (step S65). The high-precision moderepresents a mode where the resolution is the same as in the charactermode, but the sharpness factor is relatively reduced.

[0070] If the determined result in step S64 is NO, then the processflows goes to step S66 where a character/photo mode (200 dpi in thedirection of main scan and 400 dpi in the direction of sub-scan),judging from that the original document is a mixed image containing agradation image at a relatively high rate and a character area as well.

[0071] By setting one of the modes following the above process flow,satisfactory images can be reproduced in an automatic manner from anykinds of original documents.

[0072] When this embodiment is applied to color copying machines, theabove processing procedure may be modified by, for example, increasingan amount of added black (i.e., a ratio of the black component producedin a UCR process) in step S62 and reducing the amount of added black instep S65.

[0073] Also, the above processing procedure may be executed in a defaultstate of the apparatus. In this case, a means for inhibiting the aboveprocessing procedure may be provided, allowing the operator toselectively execute the above processing procedure.

[0074] Further, while the resolution is changed in the above embodimentby switching the period of a triangular wave, it may be changed byswitching, as an alternative, the size of a dither matrix.

[0075] (Other Embodiments)

[0076] The above embodiments have been described in connection with, byway of example, an application to copying machines. However, the presentinvention may be applied to an independent printer connected to a hostcomputer, etc., or a system comprising a host computer, an image scannerand a printer.

[0077] It is needless to say that the object of the present inventioncan also be achieved by supplying, to a system or apparatus, a storagemedium which stores program codes of software for realizing the function(processing procedure) of any of the above-described embodiments, andcausing a computer (or CPU and MPU) in the system or apparatus to readand execute the program codes stored in the storage medium.

[0078] In such a case, the program codes read out of the storage mediumserve in themselves to realize the function (processing procedure) ofany of the above-described embodiments, and hence the storage mediumstoring the program codes constitutes the present invention.

[0079] Storage mediums for use in supplying the program codes may be,e.g., floppy disks, hard disks, optical disks, photo-magnetic disks,CD-ROM's, CD-R's, magnetic tapes, nonvolatile memory cards, and ROM's.

[0080] Also, it is a matter of course that the function (processingprocedure) of any of the above-described embodiments is realized notonly by a computer reading and executing the program codes, but also byan OS (Operating System) or the like which is working on the computerand executes part or whole of the actual process to realize the function(processing procedure). Thus, the latter case is naturally involved inthe concept of the present invention.

[0081] Further, it is a matter of course that the present inventioninvolves such a case where the program codes read out of the storagemedium are written into a memory built in a function extension boardmounted in the computer or a function extension unit connected to thecomputer, and a CPU incorporated in the function extension board or unitexecutes part or whole of the actual process in accordance withinstructions from the program codes, thereby realizing the function(processing procedure) of any of the above-described embodiments.

[0082] As described hereinabove, according to the present invention,since resolution is controlled depending on the size of an image to beformed, it is possible to form images having satisfactory quality evenwhen observed from a close distance.

[0083] It is to be understood that the present invention is not limitedto the above-described embodiments, but may be modified or applied invarious ways within the scope of the claims.

What is claimed is:
 1. An image forming apparatus for forming a visibleimage on a predetermined recording medium based on input digital imagedata, said apparatus comprising: image forming means capable of formingvisible images with multiple degrees of resolution, and control meansfor controlling said image forming means to select one of said multipledegrees of resolution depending on the size of an image area to beformed by said image forming means.
 2. An image forming apparatusaccording to claim 1 , wherein said control means controls said imageforming means to select one of said multiple degrees of resolutiondepending on the size of a recording medium.
 3. An image formingapparatus according to claim 2 , wherein said control means controlssaid image forming means to select higher one of said multiple degreesof resolution as the size of a recording medium is reduced.
 4. An imageforming apparatus according to claim 1 , further comprising: documentreading means, and designating means for designating an effective areain a document read by said document reading means, said control meanscontrolling said image forming means to select one of said multipledegrees of resolution depending on the size of the effective areadesignated by said designating means.
 5. An image forming apparatusaccording to claim 4 , wherein said control means controls said imageforming means to select higher one of said multiple degrees ofresolution as the effective area designated by said designating means isreduced.
 6. An image forming apparatus according to claim 1 , furthercomprising: document reading means, and preparing means for preparing ahistogram of density distribution in a document read by said documentreading means, said control means controlling said image forming meansto select one of said multiple degrees of resolution depending on thedensity distribution given by said prepared histogram.
 7. A method ofcontrolling an image forming apparatus for forming a visible image on apredetermined recording medium with selected one of multiple degrees ofresolution based on input digital image data, said method comprising thestep of: controlling said image forming apparatus to select one of saidmultiple degrees of resolution depending on the size of an image area tobe formed by said image forming apparatus.
 8. A method of controlling animage forming apparatus according to claim 7 , wherein said controllingstep controls said image forming apparatus to select one of saidmultiple degrees of resolution depending on the size of a recordingmedium.
 9. A method of controlling an image forming apparatus accordingto claim 8 , wherein said controlling step controls said image formingapparatus to select higher one of said multiple degrees of resolution asthe size of a recording medium is reduced.
 10. A method of controllingan image forming apparatus according to claim 7 , further comprising thesteps of: reading a document, and designating an effective area in adocument read in said document reading step, said controlling stepcontrolling said image forming apparatus to select one of said multipledegrees of resolution depending on the size of the effective areadesignated in said designating step.
 11. A method of controlling animage forming apparatus according to claim 10 , wherein said controllingstep controls said image forming apparatus to select higher one of saidmultiple degrees of resolution as the effective area designated in saiddesignating step is reduced.
 12. A method of controlling an imageforming apparatus according to claim 7 , further comprising the stepsof: reading a document, and preparing a histogram of densitydistribution in the document read in said document reading step, saidcontrolling step controlling said image forming apparatus to select oneof said multiple degrees of resolution depending on the densitydistribution given by said prepared histogram.
 13. A recording mediumstoring a program used for forming a visible image based on inputdigital image data, wherein said program is executed through the stepsof: providing the size of an image area to be formed, and controllingresolution of a visible image to be formed depending on the size of saidimage area.
 14. A recording medium according to claim 13 , wherein thesize of said image area is defined by the size of a recording medium.15. A recording medium according to claim 14 , wherein the resolution ofsaid visible is controlled to be higher as the size of said recordingmedium is reduced.